Government regulations require proper adjustment of vehicle brakes, the adjustment being usually measured by movement of a pushrod which actuates cam devices, which in turn expand brake shoes against a brake drum of the wheel to be braked. Brake operation results in brake wear, and an increase in pushrod travel or stroke results from brake lining wear, drum wear, component strain and drum expansion at high temperature. A need for brake stroke adjustment is usually indicated by excessive pushrod travel, and there are many prior art monitoring devices which are responsive to excessive pushrod travel to indicate a need for stroke adjustment to a vehicle operator. Some of the prior art devices can be classified into several groups as below.
Some monitors utilize an electrical sensor switch which is actuated following excessive pushrod travel and generates an alarm signal, e.g. by illuminating a light or emitting a warning sound, etc. The switch has normally open switch contacts which are closed when a threshold of pushrod travel is attained. While such devices are simple, they are dependent on a functioning electrical circuit to operate the alarm. If there were an inadvertent break in the electrical circuit, or a poor electrical contact between the contacts of the switch, the monitoring alarm would not be actuated, and the vehicle operator would not be aware that the brakes were out of adjustment. Such occurrences can result from broken wires or corroded connections due to heavy vibration and/or exposure of sensitive portions of the device to contamination from dirt and moisture. Thus, this type of device is not considered to be "fail safe", and therefore undesirable as it may create a false sense of security even when inoperative. Devices of this general type are found in U.S. Pat. No. 5,226,509 (Smith) and U.S. Pat. No. 4,800,991 (Miller).
Other types of monitors utilize magnetic proximity devices which detect excessive brake pushrod travel when one component of the proximity device passes close to a cooperating component of the device. As in any magnetic proximity switch application, accurate control of spacing between the two cooperating components of the device is necessary, and this can be difficult to attain in the harsh environment of air brakes used on trucks which are exposed to wide temperature fluctuations, heavy vibration, thrown rocks, and contaminants such as dirt and moisture. Devices of this general type are shown in U.S. Pat. No. 4,937,544 (Herman) which uses a proximity dependent oscillator, and U.S. Pat. No. 4,757,300 (Sebalos) which uses a proximity or magnetic switch arrangement.
Other types of monitors have been used, for example U.S. Pat. No. 5,689,231 (Olson) discloses a rotation sensor device mounted to detect relative rotation between the pushrod and an arm of a slack adjuster cooperating with the brake cams. The sensor device may involve rather elaborate circuitry and high costs.
Most of the devices disclosed above would not necessarily indicate to the operator that there is a failure in the monitoring system, and thus the operator would be unaware that the brake adjustment was not being monitored, and could operate the vehicle with out of adjustment brakes without any warning. Other devices are relatively complex and require installation of costly new equipment for each wheel, and this discourages installation due to high installation and acquisition costs. In addition, some devices require complex maintenance routines involving specific technical skills which may also discourage their use.
Automatic slack adjusters have been developed to simplify brake stroke adjustment, but it has been found that most automatic slack adjusters also require proper and regular maintenance to ensure continued operation, and the maintenance itself is considered, by some operators, to be excessively time consuming. Without proper maintenance, automatic slack adjusters can become inoperative, resulting in the vehicle being operated with out of adjustment brakes.